In July and November 1913 Danish physicist Niels Bohr published "On the Constitution of Atoms and Molecules," Philosophical Magazine. S. 6, Vol. 26 (1913) 1-25, 476-502, 857-875. Bohr's three part paper began modern theories of the atom incorporating quantum mechanics. In the paper he postulated the existence of stationary states of an atomic system whose behavior may be described in terms of classical mechanics, while the transition of the system from one stationary state to another represents a non-classical process accompanied by emission or absorption of one quantum of homogeneous radiation whose frequency is connected with its energy by Planck’s equation. Bohr introduced the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. Bohr received the Nobel Prize for physics in 1922 for his study of the structure of atoms and of the radiation which emanates from them, as enunciated in this three-part paper.
“Atoms had been postulated in ancient times. As the year 1913 began, almost unanimous consensus had been reached, after much struggle, that atoms are real. Even before that year it had become evident that atoms have substructure, but one one yet knew by what rules their parts moved. During that year, Bohr, fully conscious that these motions could not possibly be described terms of classical physics, but that it nevertheless was essential to describe a link between classical and quantum physics, gave the first firm and lasting direction toward an understanding of atomic structure and atomic dynamics. In that sense he may be considered the father of the atom” (Pais, Niels Bohr’s Times  152).
Mehra & Rechenberg, Historical Development of Quantum Theory 1, 189-92. Pais, Niels Bohr’s Times (1991) 149-55.